1. Field of the Invention
The field of the invention relates generally to lean burn engine control, and more specifically to determining when to terminate lean operation.
2. Background of the Invention
Lean burn operating engines utilize emission control devices coupled to the engine to store NOx while operating lean, and then to reduce stored NOx when the engine operates rich.
The determination of when to operate the engine rich and terminate the lean combustion can be based on various methods. In one approach, described in EP 598917, the amount NOx stored in the device is estimated based on the amount of NOx generated in the engine. When this estimate of NOx stored reaches a predetermined value, the engine is transitioned from lean to rich.
Another approach is described in Katoh et al. (U.S. Pat. No. 5,483,795) where the amount of NOx per mile exiting the tailpipe is used to end lean operation and transition to rich.
The inventors of the present invention have recognized a disadvantage with such approaches in certain situations. In particular, if solely conditions in or downstream of the catalyst are utilized, certain situations can cause excessive NOx emissions since these set points are de-coupled from engine operation. For example, the inventors herein have recognized that during a tip-in operation from idle conditions, a high NOx and higher space velocity flow is generated. At a relatively low vehicle speed, even a relatively empty NOx trap can still emit a large tailpipe NOx spike under such high NOx and space velocity conditions.